Copper plating



United States Patent COPPER PLATING Henry G. McLeod, St. C-atharines, Ontario, Canada, and Donald A. Swalheiin, Lewiston, N.Y., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application October 24, 1956 Serial No. 617,912

1 Claim. (Cl. 204-52) This invention is concerned with electroplating copper from a cyanide copper plating bath and relates particularly to the control of hexavalent chromium in the bath.

The presence of a few parts per million of chromium in the hexavalent (Crstate in cyanide copper plating baths results in copper deposits having highly objectionable haze often accompanied by non-adherence of subsequent electrodeposited metal such as copper or nickel or the like. The presence of as little as 20 parts per million of hexavalent chromium renders the cyanide copper plating bath inoperable and useless because of the severe blotchy deposits. Fifty parts per million hexavalent chromium stops the electrodeposition of copper entirely and the bath becomes inoperative.

In many commercial electroplating operations, especially where bright work is produced, the operators follow a sequence of plating copper, then nickel and finally chromium. The racks carrying the work undergoing plating are cycled through the sequence of baths and it is next to impossible to avoid slight cross-contamination. Thus small amounts of hexavalent chromium find their Way back into the cyanide copper plating bath from the chromium plating bath. Some operators strip their racks of chromium by electrolytic means before sending them through a fresh plating cycle; others use different sets of plating racks for the copper and nickel baths and the chromium bath and this necessitates transferring the work from one set of racks to another. However, for economic reasons such expedients are very often impractical.

In some cases operators of copper cyanide electroplating baths have added small amounts of sodium hydrosulfite at regular intervals to the copper plating baths and to copper strike baths to counter the effect of hexavalent chromium. This addition will reduce the chromium to a lower valence in which state it has very little effect on the copper plate. But unfortunately the hydro sulfite can also reduce the copper cyanide to finely divided spongy copper metal and this will result in seriously rough copper deposits which are very objectionable. The general eifect of sodium hydrosulfite addition unless made in small amounts and with considerable care is to disturb the balance of composition in the cyanide copper plating bath necessary for etfective copper plating. Further, since many modern cyanide copper plating baths must be operated essentially sulfite free because of very harmful dullness produced by the S sulfur, sodium hydrosulfite can not be used in these baths or the copper strikes ahead of them to control hexavalent chromium contamination.

Another method of countering the baneful efiect of hexavalent chromium is the use of complexing or sequestering additives such as ethylene diamine tetra-acetic acid or its alkali metal salt. Unfortunately, such a sequestering agent tends to accumulate in the plating bath with a resultant slow build-up of chromium. The result is often an imbalanced and difiiculty controllable plating solution which may get out of hand easily and make continuous efiective plating at high speed virtually impossible. Furthermore, rather large amounts of the sequestering agent are necessary to mask small amounts of chromium and this organic matter has generally undesirable efiects in the plating bath.

It is accordingly an object of the present invention to provide an improved procedure for plating copper from a cyanide copper bath. A further object is the provision of a copper cyanide bath free from objectionable sulfur and organic sequestering agents while at the same time inhibiting the undesirable efiects of hexavalent chromium. Another object is to provide for a simple additive whereby the hexavalent chromium is reduced to a harmless lower valency state. Further objects will be apparent from the following discussion of the invention.

In accordance with these objectives we have discovered that the addition of stannous tin, preferably added as sodium stannite, will completely remove the chromate contamination problem associated with copper cyanide electroplating baths. The reaction appears to be essentially a reduction of the hexavalent chromium to trivalent chromium with the equivalent oxidation of stannite tin to stannate tin. One gram of chromium in the hexavalent formrequires about 3% grams of stannous (Sn+ tin to be reduced to trivalent chromium (Cr Trivalent chromium precipitates out of the plating bath.

Excess of tin as sodium stannite within moderate concentrations has no adverse effects on the plating operation. The stannite does not appear to react with any of the main components of the cyanide copper plating bath. A large excess of tin compound in the bath will be slowly removed by codeposition as metal along with the copper plate. In general neither sodium stannite nor its oxidation products are harmful to cyanide copper plating baths.

It has been discovered that sodium stannite in conjunction with the presence of small amounts of addition agents such as inorganic selenide combined with the naphthalene sulfonic acid-formaldehyde condensation product in copper plating baths improves the brightness of the resultant electrodeposits of copper.

The use of stannite in cyanide copper plating baths has the further advantage that the reduced chromium in the trivalent state is insoluble in the bath provided compleXing agents such as Rochelle salts are absent. The precipitated chromium compounds can be removed from the bath by filtration and chromium build-up thus prevented.

It has been found the stannite tin is uniquely eflective for the inactivation and removal of hexavalent chromium. Not all reducing agents are effective to convert hexavalent chromium to the trivalent insoluble form. Thus sodium hypophosphite is generally a good reducing agent but not effective to reduce the chromium. Also, other reducing agents such as sodium hydrosulfite, potassium antimonyl tartrate and others will leave behind in the bath undesirable contamination. Unexpectedly it was thus discovered that not only does stannous tin reduce hexavalent chromium but that it can be used without introducing undesirable contamination into the cyanide copper plating bath.

Stannous tin may be added in any convenient form but it must be soluble in the bath to be effective. A preferred method is to add the required amount of stannous chloride to a sodium hydroxide solution to produce a solution of sodium stannite which can then be added to the plating bath. Accordingly in the further discussion of this invention reference will be made to the addition of stannite tin as stannous chloride with the understanding that the stannous chloride will first be dissolved in aqueous sodium hydroxide. A preferred concentration of sodium hydroxide will be between about 2 to by weight although other concentrations may, if desired, be used.

Generally the beneficial effects of sodium stannite will be apparent in a few minutes, often as little as five minutes after addition to the platingbath.

.Thezfollowingexamples will illustratethepractice of our invention but it is to be understood that the invention is not limited by the specific detailsthus disclosed.

Example 'I A sample of an operating cyanide copper plating bath was placed in a standard Hull cell and a plating test was .made on a buffed brass panel. This test showed that the bath produced a bright smooth copper plate. The composition of thisbath .was approximately:

Ounce/ gallon Copper cyanide c 11 Free potassium cyanide 2 Potassium hydroxide 2 Potassium carbonate 2 This bath also contained about 15 parts per million of cuprous selenide and 600 parts per million of the naphthalene sulfonic acid-formaldehyde condensation product.

To another sample of this operating bath there was added 10 parts per million of hexavalent chromium as sodium bichromate. Plating with this bath produced a blue-violet haze over approximately two-thirds of the bright plating range. Addition of about 50 parts per million of stannous chloride freshly added to sodium hydroxide solution completely removed the blue-violet haze and the bath now again produced a smooth copper plate which was even brighter than the plate from the untreated copper cyanide operating bath. Addition of a large excess of stannous chloride, 600 p.p.m. had no harmful effect and actually improved the brightness and the width of the plating range.

Example 11 Using another sample of the same operating solution as in Example I, a series of Hull cell panels was run. First p.p.m. hexavalent chromium was added to the bath. Typical blotchy plates due to chromate contamination appeared. Sodium stannite was added step-wise to this solution. 20 ppm. stannous chloride removed a small amount of the contamination with a slight improved plate. 50 ppm. and 80 p.p.m. removed progressively more of the ill effects of the contamination. When 100 p.p.m. had been added, the undesirable effects of the chromate contamination had disappeared completely.

Example III A commercially operated copper strike bath containing about 3 oz./ gal. of copper metal and with a free sodium cyanide of about 1 oz./gal. was observed to be depositing copper on which dark draining streaks could be seen. Such streaks are usually considered to be an indication of a small amount of hexavalent chromium in the strike bath. 0.004 oz./ gal. of stannous chloride added as sodium stannite was added to the strike bath. In a .few minutes the dark streaks disappeared. Operating personnel regarded this as a convincing demonstration of the utility of stannous tin in controlling chrome contamination in the copper strike and the process could be continued in successful operation.

Example IV A large commercial copper plating tank was plating semi-bright copper with a pronounced pink haze on it due to hexavalent chromium contamination. 0.0025 oz./ gal. of stannous chloride converted to sodium stan- 4 nite was added to this bath. Work coming from the tank showed a noticeable improvement in brightness within 5 minutes. The pink haze was completely gone from the work and full brightness restored in less than 20 minutes.

Example V A Hull cell panel plating test was made on a sample of a commercially operating cyanide copper plating solution. The panel showed the copper bath to be in excellent condition with no chromate contamination. Several hundred p.p.m. of stannous chloride added as sodium stannite was added to the plating bath. A Hull cell plating test again showed the solution to be plating even a better bright copper. A drop of chrome plating solution was taken from a near-by operating plating tank and added directly to the Hull cell (equivalent to adding about 1 ml. chrome plating solution to a gallon of copper solution). A Hull cell panel test was made immediately and it showed that the bath still produced an excellent bright deposit with no traces of chromate contamination.

As to cyanide copper plating baths our invention is not limited to the particular composition shown in Example I. Copper plating baths with dilute compositions suitable for copper strike as well as more concentrated baths suitable for heavy electrodeposition of copper can be treated with stannous tin to eliminate or mask the undesirable effects due to the presence of hexavalent chromium. These baths preferably contain potassium compounds but they may also contain sodium compounds in addition to or be made up entirely of sodium compounds.

Stannous tin may be added in any desired for-n1 provided only that it be sufliciently soluble in the bath to be capable of fairly rapid reaction with the hexavalent chromium. The preferred method of adding the tin comprises dissolving the required amount of solid stannous chloride in excess caustic soda or potash solution and then adding this solution in which the tin is present essentially as sodium or potassium stannite to the cyanide copper bath to be treated.

Stannous tin may be added to the cyanide copper plating bath before contamination with chrominum has occurred or after such contamination. When contamination can take place it is good practice to keep a small concentration of stannous tin in the bath at all times and thereby avoid the inadvertent production of objectionable copper plates that results from hexavalent chromium contamination. In general the stannous tin must be present in sufficient amount to reduce substantially all the chromium from the hexavalent to the trivalent state. In any case it is essential that hexavalcnt chromium never be permitted in the bath in excess of 5 parts per million. Preferably the stannous tin should always be present in excess of that required to reduce all the hexavalent chromium. Indeed tin may be present in as large an excess as 1000 ppm. to that required to reduce the chromium.

Tin in the copper cyanide plating bath both in the stannic or the stannous form acts as a brightener and improves the appearance of the copper plate. Tin is especially effective as a beneficial agent in the bath when simultaneously present with small amounts of inorganic selenide or the naphthalene sulfonic acid-formaldehyde condensation product or both. Indeed the presence of all three agents together is especially effective in pro moting the deposition of smooth, bright copper deposits from the cyanide copper plating solutions.

The selenium to be effective must be present as an inorganic selenide, preferably as copper selenide. The selenium in this valence state of minus 2 to be effective must be present to the extent of at least 1 ppm. and may be present up to 20 ppm. Generally, a concentration above about 40 p.p.m. is objectionable.

.It has been found that the condensation product of naphthalene sulfonic acid and formaldehyde to be effective should be present in amount of at least 200 p.p.m. and preferably at least 400 ppm. in the cyanide copper plating bath. For economic reasons amounts of this agent is excess of 3000 ppm. will not be used and are not necessary to achieve the desired effects on copper plating quality.

What we claim is:

In a process for the electrodeposition of bright copper deposits from an aqueous cyanide copper plating bath containing hexavalent chromium ions, in a concentration in excess of 5 p.p.m., the step of maintaining a sufficient amount, less than 1,000 p.p.rn., of stannous tin ions in said bath to prevent the concentration of said chromium ions from exceeding 5 ppm.

References Cited in the file of this patent UNITED STATES PATENTS 1,596,300 Otis et a1 Aug. 17, 1926 2,658,032 Faust et al. Nov. 3, 1953 2,701,234 Wernlund Feb. 1, 1955 2,770,587 Ostrow Nov. 13, 1956 OTHER REFERENCES Diggin: Monthly Review of American Electroplaters Society, May 1946, page 517.

Safranek et al.: Plating, volume 41, Number 10, October 1954, pages 1159-1160.

Meyer et al.: Transactions Electrochem. Soc., vol.

15 73 (1938), pages 384, 403, 405 and 406. 

